HP-LP BYPASS SYSTEM The HP-LP bypass system is such an important system that any body whoever wants to start the turbine should understand its functions, its necessity and the procedure to keep it in service along with the interlocks for keeping the system and to maintain the system to continue in service till its function is over during the process of warming up the turbine and rolling the turbine. When once we are familiar to keep the HP/LP bypass system into service the balance process of turbine operation is very easy and it is almost that the theory/process of turbine operation is totally understood by us, because all the turbine systems will be in service by the time the HP-LP bypass system is taken into service. As seen from the figure the main turbine system i.e. HP-IP-LP turbines are bypassed and the boiler is operated as though the turbine system is in service for all practical purposes. Purpose of HP/LP bypass systems: To provide reheater protection by establishing steam flow through reheater while going for coal firing to raise the boiler parameters. To achieve turbine matching steam parameters during hot/warm start up operations. To warm up the main steam, cold reheat and hot reheat steam pipe lines and to facilitate warming up of ESVs, IVs, HPT casing and IPT casing during all types of turbine start up operations. Pre conditions for keeping HP/LP bypass system in to service: a) The condenser vacuum should be above 540 mm of Hg. b) The bypass valves opening position shall be not less than 2%. c) The condensate dumping valves are kept opened. d) The CRH NRVs and their bypass valves should be in closed position. e) The oil supply units for the operation of the bypass valves shall be in service. f) The Generator transformer circuit breaker shall be in open position. Functions of HP-LP bypass system: a) Main steam from boiler outlet is admitted into cold reheat lines through HBP-1 & HBP-2, thus bypassing the HP turbine. b) The temperature of steam entering the cold reheat lines was reduced to below 3800 C by the HP bypass spray water injection valves HBPE-1 & HBPE-2 (Feed water pressure was reduced to 90 kg/cm2 by the ‘BD’ valve). c) Then the steam undergoes reheat cycle in the boiler and comes back to hot reheat lines. d) HRH steam is admitted into condenser through LBP-1 & LBP-2, thus bypassing the IPT & LPT. e) Temperature of steam entering the condenser was reduced to less than 2000C by the LP bypass spray water injection valves LBPE-1 & LBPE-2. (Water drawn from the CEP discharge header was directly used through LBPE-1 & LBPE-2). Note: The HP- LP bypass system is always kept in Manual mode to avoid malfunctions of the connected systems. Modes of operation of HP/LP bypass system: The HP/ LP bypass system can be operated either by pressure mode or position mode operation. In the pressure mode operation, the HP bypass valve will open in auto for the pre set pressure which will be above the normal operating pressure. The LP bypass valve will open in auto on interlock, accordingly to maintain the pre set pressure P- set value in the reheat header. In the position mode operation the LP bypass valve will open in auto according to the percentage opening of the HP bypass valve. Description of HP-LP bypass console: There will be one console on the control desk for the HP-LP bypass system wherein there are instruments which can be set for opening the HP bypass & LP bypass valves as per the pressure set points. Both the set pressures and actual pressures can be read on the instruments. Further, there will be temperature indicators with set points indicating both the set temperatures and the actual temperatures for the HP bypass down stream steam temperature and also for the LP bypass down stream steam temperatures. The percentage openings of both HP- LP bypass valves and their spray valves can also be read from the instruments on the console. Note: Feed water tapped from the BFP discharge header is used for the HP bypass spray and the condensate tapped from the CEP discharge header is used for the LP bypass spray. Roll of HP-LP bypass system during emergencies: It shall be remembered that during normal operation of the unit the HP-LP bypass system will not be in service and it is required to come into service in auto in the event of heavy Load throw offs on the unit or in the event of unit tripping to save the unit by bringing it on house load operation by providing sufficient steam bypass without affecting the boiler operation & operation of mills, so that the turbine can be reloaded/restarted immediately. Thus lot of saving in time is obtained to bring back the unit to full load operation and restore normalcy. Keeping the HP bypass system in service: The pressure set pint of HP bypass valves (L & R) –‘P-set’ is to be brought down to below the actual MS pressure. Then the HBP-1 & HBP-2 valves will open in auto. Care should be taken to see that the actual MS pressure will not fall below the P-set pressure; otherwise the HBP-1 & HBP-2 get closed interrupting the steam flow to the reheater and allowing the boiler to trip on reheater protection. Hence, to continue the status of HP bypass valves in open position, the boiler pressure shall be closely monitored and maintained to be always above the P-set pressure of the HBP-1 & HBP-2, maintaining the mill loading accordingly. Observations after opening the HP bypass valves: a) CRH pressure will be observed on the recorder on the turbine panel. If computerized operation is available the CRH pressure can be read on the screen along with its trend to monitor. b) HP bypass down stream temperatures can be read on TBP-1 & TBP-2. c) BD valve- the main HP bypass spray valve & the individual spray valves HPE-1 & HBPE-2 will start opening in auto to maintain the reheat steam temperature always less than 3800 C. Keeping the LP bypass station in service: The moment the HP bypass valves have opened the LP bypass valves also will open in auto to maintain the reheat header steam pressure as per the P-set value. Observations after opening the LP bypass valves: a) The LP bypass valves LBP-1 & LBP-2 and their spray valves LBPE-1 & LBPE-2 will start opening in auto immediately after opening of the HP bypass valves. b) LBP down stream temperatures (L& R) will be indicated on the instruments TLBP-1 & TLBP-2 on the console. The temperatures should always be less than 2000 C as already set by temperature setting T-set. c) There will be slight rise in the LPT exhaust hood temperature as indicated on the turbine panel or in the turbine parameters as indicated on the CRT. d) There will be slight fall in the condenser vacuum also and then stabilizes to normal value slowly. Interlocks for HP-LP bypass system: HP-LP bypass system gets isolated if; a) The condenser vacuum falls to less than 540 mm of Hg. b) HBP-1 & HBP-2 valves openings are less than 2%. c) HP bypass down stream temperatures rise to more than 3800 C. d) LP bypass temperatures rise to more than 2100 C. Actual procedure of taking HP-LP bypass system into service: 1) Open up stream drains of HP bypass & LP bypass systems and down stream drains of HP bypass systems to turbine flash tank. 2) Open LP bypass system drains to condenser. 3) Open all CRH & HRH lines drains to turbine flash tank. 4) Open LP bypass vales 1& 2 for evacuating the condensate if any in the LP by pass down stream lines to avoid hammering and then close them before opening the HP bypass valves. 5) Bypass the protection of HP-LP bypass trip to condenser vacuum low to avoid unnecessary tripping of the system for minor condenser vacuum variations. 6) Open HP bypass valves first and then LP bypass valves manually and observe HRH steam pressure and CRH steam flow. Check up the opening of HP-LP bypass system spray valves in auto and ensure that the HRH down stream temperatures are less than 3800 C and the LP bypass system down stream temperatures are less than 2100 C. Effects of operation of HP-LP bypass system: The turbine system or any system is so sensitive and the effect of any operation shall be thoroughly satisfied not to go wrong and achieve progress in the process of operation. Hence, after keeping the HP-LP bypass system in service the following observations will confirm for the satisfactory functioning of the system: 1) Steam flow is established in the MS, CRH & HRH steam lines which can be witnessed from the recording charts or on the CRT. 2) Steam parameters will be increasing at the boiler outlet. This can be verified from both charts and indicating instruments. [Note: The following shall be taken care of, after taking the HP-LP bypass system into service: a).Difference of pipe metal temperatures shall not be more than 150 C between L & R side of the pipes of MS, CRH & HRH steam. b) The rate of rise of pipe metal temperatures should not exceed 20- 300 C for every 5 minutes.] One shall be thorough with the sequence of operations and shall be alert to start the sub systems and keep them ready for starting the main system. Here, the main important subsystem is -condenser vacuum system- which should have been kept ready in time by the time the boiler systems require the mill to be started to proceed with further pressure raising operations. For pulling condenser vacuum, the steam jet ejectors are required to be kept in service for which auxiliary steam is required. Hence, the auxiliary steam header also needs to be charged prior to the requirement of ejectors for pulling vacuum. It shall be remembered in general wherever the steam is to be admitted, the line drains in the system shall be opened before charging it and also that the admitting steam temperature shall be at least 50 deg. C higher than that of the section /system into which the steam is going to be admitted. Similarly the milling system shall be made ready with all preliminary checks so that the mill can be taken into service immediately after taking the HP-LP bypass system into service so as to proceed without any delay to raise the steam parameters. Further by the time the steam parameters are attained the drain valves of steam pipe lines or of the systems which are to be warmed up shall be kept opened and the steam admitted early.
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